mac_veriexec: add mac_priv_grant check for NODEV

Allow other MAC modules to override some veriexec checks.

We need two new privileges:
PRIV_VERIEXEC_DIRECT	process wants to override 'indirect' flag
			on interpreter
PRIV_VERIEXEC_NOVERIFY	typically associated with PRIV_VERIEXEC_DIRECT
			allow override of O_VERIFY

We also need to check for PRIV_VERIEXEC_NOVERIFY override
for FINGERPRINT_NODEV and FINGERPRINT_NOENTRY.
This will only happen if parent had PRIV_VERIEXEC_DIRECT override.

This allows for MAC modules to selectively allow some applications to
run without verification.

Needless to say, this is extremely dangerous and should only be used
sparingly and carefully.

Obtained from:	Juniper Networks, Inc.

Reviewers: sjg
Subscribers: imp, dab

Differential Revision: https://reviews.freebsd.org/D39537

sys/security/mac_veriexec/mac_veriexec.c

@@ -51,6 +51,7 @@
 #include <sys/sysctl.h>
 #include <sys/vnode.h>
 #include <fs/nullfs/null.h>
+#include <security/mac/mac_framework.h>
 #include <security/mac/mac_policy.h>
 
 #include "mac_veriexec.h"
@@ -430,6 +431,18 @@ mac_veriexec_priv_check(struct ucred *cred, int priv)
 	return (0);
 }
 
+/**
+ * @internal
+ * @brief Check if the requested sysctl should be allowed
+ *
+ * @param cred         credentials to use
+ * @param oidp         sysctl OID
+ * @param arg1         first sysctl argument
+ * @param arg2         second sysctl argument
+ * @param req          sysctl request information
+ *
+ * @return 0 if the sysctl should be allowed, otherwise an error code.
+ */
 static int
 mac_veriexec_sysctl_check(struct ucred *cred, struct sysctl_oid *oidp,
     void *arg1, int arg2, struct sysctl_req *req)
@@ -533,6 +546,9 @@ mac_veriexec_check_vp(struct ucred *cred, struct vnode *vp, accmode_t accmode)
 				return (error);
 			break;
 		default:
+			/* Allow for overriding verification requirement */
+			if (mac_priv_grant(cred, PRIV_VERIEXEC_NOVERIFY) == 0)
+				return (0);
 			/*
 			 * Caller wants open to fail unless there is a valid
 			 * fingerprint registered.

sys/security/mac_veriexec/veriexec_fingerprint.c

@@ -42,11 +42,14 @@
 #include <sys/malloc.h>
 #include <sys/mount.h> 
 #include <sys/mutex.h>
+#include <sys/priv.h>
 #include <sys/proc.h>
 #include <sys/sbuf.h>
 #include <sys/syslog.h>
 #include <sys/vnode.h>
 
+#include <security/mac/mac_framework.h>
+
 #include "mac_veriexec.h"
 #include "mac_veriexec_internal.h"
 
@@ -292,7 +295,8 @@ mac_veriexec_fingerprint_check_image(struct image_params *imgp,
 
 	case FINGERPRINT_INDIRECT: /* fingerprint ok but need to check
 				      for direct execution */
-		if (!imgp->interpreted) {
+		if (!imgp->interpreted &&
+		    mac_priv_grant(td->td_ucred, PRIV_VERIEXEC_DIRECT) != 0) {
 			identify_error(imgp, td, "attempted direct execution");
 			if (prison0.pr_securelevel > 1 ||
 			    mac_veriexec_in_state(VERIEXEC_STATE_ENFORCE))
@@ -326,6 +330,23 @@ mac_veriexec_fingerprint_check_image(struct image_params *imgp,
 		identify_error(imgp, td, "invalid status field for vnode");
 		error = EPERM;
 	}
+	switch (status) {
+	case FINGERPRINT_NODEV:
+	case FINGERPRINT_NOENTRY:
+		/*
+		 * Check if this process has override allowed.
+		 * This will only be true if PRIV_VERIEXEC_DIRECT
+		 * already succeeded.
+		 */
+		if (error == EAUTH &&
+		    mac_priv_grant(td->td_ucred, PRIV_VERIEXEC_NOVERIFY) == 0) {
+			error = 0;
+		}
+		break;
+	default:
+		break;
+	}
+
 	return error; 
 }
 

sys/sys/priv.h

@@ -520,10 +520,16 @@
  */
 #define	PRIV_KDB_SET_BACKEND	690	/* Allow setting KDB backend. */
 
+/*
+ * veriexec override privileges - very rare!
+ */
+#define	PRIV_VERIEXEC_DIRECT	700	/* Can override 'indirect' */
+#define	PRIV_VERIEXEC_NOVERIFY	701	/* Can override O_VERIFY */
+
 /*
  * Track end of privilege list.
  */
-#define	_PRIV_HIGHEST		691
+#define	_PRIV_HIGHEST		702
 
 /*
  * Validate that a named privilege is known by the privilege system.  Invalid